Gas absorption spectroscopy generally measures the presence and/or concentration of a species of interest in a gas sample by passing a light beam through the sample and detecting the absorption at wavelengths of a particular spectral absorption feature of the species of interest. Generally, such a feature is an absorption line that represents the frequency of light corresponding to vibrational, rotational or electronic transitions of molecules of the gas of interest. Tunable diode lasers provide many advantages for such gas absorption spectroscopy measurements in that the lasers can be tuned to the center of a spectral feature and generate a narrow signal relative to the width of the spectral feature.
Laser absorption spectroscopy can thus offer high speed and relatively high precision capabilities for detecting a variety of trace gas species in gas samples at atmospheric pressures with relatively low cross sensitivity to other gas species or components. Tunable diode laser spectrometers are particularly suited to high sensitivity studies, in part, because they may be frequency-modulated to reduce low frequency laser noise and electronic noise. In general, a laser spectrometer will include a frequency tunable laser that generates an illumination output beam which is directed through a sample cell that contains a gas sample. The output beam is then directed to an optical detector and the signal of the optical detector is demodulated to obtain an absorption induced signal. This absorption induced signal can be used to identify one or more species of interest within the gas sample.
In some applications, it is important to detect trace moisture levels in a gas sample, such as natural gas, using a tunable diode laser absorption spectrometer. In such situations, the detection may be limited by spectral interference due to atmospheric moisture. Atmospheric, or any residual moisture present in the optical paths of the spectrometer, outside the sample cell may contribute to measurement error. Since the moisture level that may need to be detected within the gas sample is often below one part per million, atmospheric moisture levels ranging from 7,000-30,000 parts per million can generate significant measurement errors. Another important limitation in the detection of trace moisture levels in a gas sample is that the laser noise and the optical noise originating near the laser beam delivery optics may also limit the sensitivity and accuracy of the spectrometer.
Providing an apparatus and method that is able to adjust or otherwise compensate the trace moisture detection of a gas sample of interest based on atmospheric moisture and/or laser source noise/optical noise originating in the laser beam delivery optics would provide tunable diode laser absorption spectroscopy with improved accuracy and sensitivity.